The RNAV type surface navigation allows an aircraft to fly from a waypoint to another waypoint, and no longer from ground stations (of radio-navigation means of the NAVAID type) to ground stations.
As known, the RNP concept corresponds to a surface navigation, for which (on board the aircraft) monitoring and warning means are added, allowing to ensure that the aircraft remains in a corridor, referred to as RNP, around a reference trajectory and authorizing taking into consideration curved trajectories. Outside this corridor, potentially relief or other aircrafts could be present. The performance required for a RNP type operation is defined by a RNP value representing half the width (in nautical miles: NM) of the corridor around the reference trajectory, in which the aircraft should remain 95% of the time during the operation. A second corridor (around the reference trajectory) of half a width twice the RNP value is also defined. The probability that the aircraft goes out of this second corridor should be lower than 10−7 per hour of flight.
The concept of RNP AR operations is still even more stringent. The RNP AR procedures are indeed characterized by:                RNP values:                    being lower than or equal to 0.3 NM in approach, and that could go down to 0.1 NM; and            being strictly lower than 1 NM at the start and during a throttling up, and that could also go down to 0.1 NM;                        a final approach segment that could be curved; and        obstacles (mountains, traffic, . . . ) that could be located at twice the RNP value with respect to the reference trajectory, while for usual RNP operations, an additional margin with respect to obstacles is provided.        
The air authorities have defined a target level of safety Target Level of Safety (“TLS”) of 10−7 per operation, whatever the type. In the case of RNP AR operations, as the RNP values can go down to 0.1 NM and the obstacles could be located at twice the RNP value of the reference trajectory, this objective results in a probability that the aircraft goes out of the half-width corridor D=2.RNP that should not exceed 10−7 per procedure.
The equipment on board aircrafts (flight management system, inertial unit, means for updating GPS data and means for guiding the autopilot), as well as the usual architecture, do not allow to reach the target level of safety, if mitigation operational means are not provided, including for detecting and managing possible breakdowns. This is why a special authorization is required for this type of operation, so as to ensure that the operational procedures and the pilots' training allow the target level of safety to be reached. Moreover, as the crew should take in charge some breakdowns, the aircrafts are to-day not able to guarantee a RNP value of 0.1 NM in a breakdown situation, as the crew are not able to meet the performance requirements in manual piloting.
On current aircrafts, the monitoring of RNP AR operations is implemented by means of two usual functions, that is:                a first function monitoring the accuracy and the integrity of the position calculation:                    the accuracy of the position is compared to once the RNP value;            the integrity is compared to twice the RNP value; and            if one of the two parameters, accuracy or integrity, exceeds the allotted threshold, a warning is emitted and the crew should take appropriate actions; and                        a second function allowing the crew to monitor the guidance of the aircraft:                    the lateral and vertical deviations of the aircraft with respect to the reference trajectory are displayed and shown to the crew;            the crew monitors the deviations compared to the budgets allotted for each deviation. If the crew detects an excessive deviation, the crew should keep the aircraft under control again and take the adequate corrective actions.                        
As set forth previously, the current aircrafts are not able to guarantee a RNP value of 0.1 NM in a breakdown situation and the crew should be trained specially for flying the RNP AR procedures. The crew should, indeed, be able to detect and process, adequately, breakdowns being able to compromise the ongoing operation.
The objective for future aircrafts is to be able to fly RNP AR procedures with RNP values up to 0.1 NM, and this without restriction (in a normal situation and in the case of a breakdown) in start, approach and throttling up phases. To this end, the crew should no longer be considered as the main means for detecting and processing breakdowns.
The ability of an aircraft to follow a RNP AR trajectory comprising at least one turn, could be comprised under certain particular conditions, including in case of unfavourable winds. Under this type of conditions and depending on the wind force being encountered, the aircraft is sometimes no longer able to follow the defined trajectory. This type of problem could also occur when the speed of the aircraft exceeds a reference speed for the turn being started. This situation could, more specifically, occur as a result of a breakdown or of an error from the crew concerning a manually selected speed.
It is thus advantageous to be able to monitor the ability of the aircraft to meet the RNP AR performance. If the required performance level is not reached, the crew should be made aware of this, so as to be able to most appropriately react.
The present invention aims at solving the above mentioned drawbacks. It relates to a method for automatically monitoring the ability of an aircraft to follow a flight trajectory comprising at least one turn.